Engine harmonic cancellation systems are active noise reduction systems that are used in motor vehicles, for example in cabins or in muffler assemblies, to reduce or cancel engine harmonic noise. Engine harmonic cancellation systems use one or more microphones as input transducers. A signal related to the noise to be canceled is also inputted to an adaptive filter. The output of the adaptive filter is applied to one or more transducers that produce sound (i.e., loudspeakers). The sound is acoustically opposite to the undesirable engine sounds that are to be canceled. The adaptive filter can alter the magnitude and/or the frequency of the input signal. The aim of the system is to cancel the microphone signal at the frequency or frequencies of interest. In order to do so, the loudspeaker outputs have a negative gain.
In certain situations these engine harmonic cancellation systems can become unstable and allow the loudspeaker sound output levels that are designed to cancel the engine noise to diverge. Such an unstable engine harmonic cancellation system can produce loud and noticeable noise artifacts. One cause of such instability can be a change in the transfer function of the vehicle cabin, which can cause the loudspeaker output gain to effectively become positive.